To print an image on a substrate, a typical digital printer first forms an electrostatic copy of the image, conventionally referred to as a “latent image”, on a photosensitive surface, for example on a cylindrical roller, hereinafter referred to as a “photosensitive imaging cylinder” (PIC). First a charger deposits a substantially uniform charge density on the photosensitive surface. The latent image is then formed by discharging regions of the charged photosensitive surface to generate a pattern of charged and uncharged pixels on the photosensitive surface that replicates the image. A developer applies ink or toner of desired color to the charged or uncharged regions using an electrostatographic process.
The toner on the PIC is then transferred from the PIC to a final substrate, optionally via an “intermediate transfer member” (ITM) to print the image. In single color printing, such as in black and white printing, the latent image is a copy (or inverse) of the image to be printed. In printing a multicolor image, such as in CMYK printing, the latent image is a copy (or inverse) of a color separation of a plurality of color separations required to print the image in color. A different color toner is transferred to the substrate for each of the plurality of color separations to print the image.
Discharging regions of the PIC's photosensitive surface to generate the latent image is generally accomplished by illuminating the regions with a beam (or multiple beams) of light from a laser that is focused to a point on the photosensitive surface. The beam is controlled so that its focal point repeatedly scans the photosensitive surface along a line parallel to the axis of the PIC as it rotates rapidly about the axis. As the beam scans a line of the photosensitive surface, it is turned on to illuminate regions of the surface along the scan line that are to be discharged and turned off so as to not illuminate regions along the scan line that are not to be discharged. The latent image is built up line by line on the photosensitive surface as the PIC turns.
To provide a latent image having accurately controlled pixel densities and consequently a printed image for which hue saturation and brightness of printed regions are accurately controlled, rotation speed of the PIC and time intervals between line scans during scanning should be substantially constant. If the rotation speed of the PIC changes, or the time interval between the onset of scans by the laser changes, spacing between scan lines will vary. As a result, the latent image will evidence bands of pixels that are parallel to the scan direction for which the pixel densities will be greater than or less than desired. When toner is applied to the PIC, the bands having greater pixel density will acquire too much toner while bands having lesser pixel density will acquire too little toner. An image printed on a substrate from the latent image will, as a result, have bands of too little or too much toner, i.e. bands of unwanted variations in optical density, that are perpendicular to the process direction of the image and quality of the printed image will be compromised. (The process direction is a direction along which the image moves during image formation or transfer.)
The term “banding” is generally used to refer to bands of undesired variations in the optical density of an image. For the situation described above, these variations are substantially perpendicular to the process direction and parallel to the scan direction. Banding or other undesired variations of density in an image along the scan direction, i.e. perpendicular to the process direction, may also occur, due to variations in scanning speed. It is relatively difficult to control all the variables that can cause banding in an image printed by digital printers and digital printer images may exhibit banding of varying degrees of severity in one or both directions.